Known interchangeable-lens, TTL light measuring type, single lens reflex cameras are generally classified into two types: one is a so-called diaphragm preferential, automatic exposure type in which a diaphragm value is preset and a shutter speed is determined according to a quantity of light: and the other is a so-called shutter speed preferential, automatic exposure type in which a shutter speed is preset and a diaphragm value is determined according to the quantity of light. Both such cameras require a so-called diaphragm interlocking mechanism for transmitting a diaphragm value from the lens to the body and vice versa, resulting in a complicated construction. Referring to a known diaphragm preferential, TTL light measuring single lens reflex camera by way of example, since the measurement of light is effected with an objective lens maintained in a fully open position, if the brightness of an object remains the same, the quantity of light incident on a light receiving element varies greatly according to the difference in aperture value at the fully open position of the particular objective lens used. For this reason, the measurement of light is not possible unless correction is made by transmitting from the lens to the body information of the aperture value in the fully open position of the objective lens. Such a camera thus requires a mechanism for transmission of information concerning the aperture value of the objective lens in the fully open position. Upon photographing, since the diaphragm is stopped down to a set aperture value, unless information of the set aperture value is transmitted from the lens to the body, a proper exposure time can not be calculated according to the value of light measured, unless the value remains intact. A diaphragm interlocking mechanism is thus needed. In addition, a diaphragm operating mechanism is necessary for stopping down the diaphragm of an objective lens to a set aperture value. Such diaphragm mechanism need be provided for every interchangeable lens; however, the provision of such mechanisms in the conventional camera leads to a costly camera.